This invention is in the field of decoders for compressed video and audio signals.
Video compression is a technique used to send or store digitized video data more compactly, so that more "movies" can pass along the same communication channel or be stored in a particular storage medium.
The Motion Pictures Experts Group (MPEG) has defined International Organization for Standardization (ISO) standards for video and audio compression. MPEG-1 is a compression technique for compact disks, and is suited for hard disks. MPEG-2 is a similar compression technique for cable TV and for high definition TV (HDTV). However, MPEG-1 and -2 really define only the compressed bitstream as it is stored or sent, as a communications protocol.
The specific video protocols and specifications for MPEG-2 are defined in a three-part document published in 1994 by ISO and the International Electrotechnical Commission as a draft international standard, No. ISO/IEC DIS 13818-1, -2, and -3. Those documents are hereby incorporated by reference and referred to herein as "Part 1," "Part 2," and "Part 3, " respectively. The audio specification is defined in another ISO/IEC document, entitled "Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/s," produced by Joint Technical Committee 1, subcommittee 29 as Doc. No. 11172-3. That document is incorporated by reference and identified as the "Audio Standard."
Special cases of MPEG-1 and -2 specify the communication rates and screen sizes. One special case of MPEG-1, known by the acronym, SIF/CPB, narrows down the video's display dimensions to about half the width and height of a TV screen, and specifies a compressed data transmission rate of 1.5 Mbits/sec. A special case of MPEG-2, known as "main level," narrows down the video's display dimensions to about the width and height of a TV screen and specifies a compressed data transmission rate of 15 Mbits/sec. Finally, the acronym "SNR," which refers to the scalability profile, means that a low-bandwidth, highly reliable bitstream can be supplemented with a high-bandwidth, low-reliability bitstream. In this profile, with some exceptions, the receiver can display low-resolution pictures sent over the highly reliable bitstream and can display high-resolution pictures using both bitstreams.
A main-level, SNR profile, MPEG-2 decoder can also decode MPEG-1 SIF/CPB small-screen format, as well as simple and non-scalable profile, low- and main-level, MPEG-2 formats. MPEG-1 SIF/CPB is currently a rapidly emerging compression technique, as large numbers of CD-ROMs are being developed for personal computer multimedia systems. MPEG-2 is anticipated as being even more important commercially. Every cable set-top converter and every satellite dish is expected to use an MPEG-2 main-level decoder. HDTV is expected to use a "high" level, MPEG-2 compression technique, resulting in the need for an MPEG-2 "high" level, MPEG decoder in every HDTV set. No integrated circuit implementing high-level MPEG-2 has been announced at this time.
In view of the foregoing, there is a continuing desire in the art for an integrated circuit that can operate as an MPEG decoder more efficiently, in particular, a main-level, SNR profile, MPEG-2 decoder. Specifically, it is desired to achieve a decoder that can use less circuit area, and consequently achieve a lower manufacturing cost, than known implementations. As with any integrated circuit, the circuit area impacts the cost of the circuit dramatically.
The known technique most commonly used for discrete cosine transforms, the Loeffler method, is not well suited to either pipelining or a simple iterative loop because it does not perform identical operations on each element. It is not suited to residue arithmetic because it executes several adds before executing the multiply, increasing the precision, and thus the number of moduli, needed by the method. The Loeffler method is explained in an article by Loeffler et al., "Practical Fast 1-D DCT Algorithms with 11 Multiplications," printed at pages 988-91 of the 1989 publication resulting from the International Conference on Acoustics, Speech, and Signal Processing, which is hereby incorporated by reference.
It is an object of the invention, therefore, to create a circuit that can perform the video decoding and transforms necessary to implement the MPEG-2 standard without undue circuit sizes.
It is another object of the invention to create an MPEG-2 decoder circuit on a single integrated circuit.
It is another object of the invention to create a low-cost, main-level, SNR profile, MPEG-2 decoder.
It is another object of the invention to create a MPEG-2 decoder circuit that has a chip area, and hence a cost, that is reduced by about an order of magnitude over other, expected implementations of MPEG-2 decoders.
It is another object of the invention to create a circuit that can perform a discrete cosine transform on video data in an efficient manner.
It is another object of the invention to create a circuit that can perform variable-length decoding in an efficient manner.
It is another object of the invention to create a video decoder circuit that operates using residue arithmetic.
It is another object of the invention to create a variable-length decoder for video data streams that uses PLAs.
It is another object of the invention to create low-cost, efficient methods for operating on video data in accordance with the MPEG-2 specifications to perform variable-length decoding and discrete cosine transforms.
It is another object of the invention to implement an efficient residue-tobinary converter.